On 18 February 1964, Sonny & Cher recorded the song. Outside of being backing vocalists, it was the couple's first professional recording together. To further cement the recording into music history, it became Sonny & Cher's first ever single.
Sonny learned from Spector to put an unimportant instrumental piece on a single's B-side, so not to take attention away from the A -side. The flip of 'The Letter' was so unimportant that its name changed from 'String Fever' to 'Spring Fever' on certain re-issues.

The duo rush-released the single under the guise of "Caesar & Cleo" on Vault Records. Despite having hit potential, the single didn't reach the US Hot 100 though it became a regional hit in California, and to a lesser degree, Connecticut and New York.

In mid-1965, Sonny & Cher - as "Sonny & Cher" - caught their big break with #1 single 'I Got You Babe'. 'The Letter' was one of four early singles (the others being 'Baby Don't Go', 'Just You' and 'Love Is Strange') to be re-issued to latch onto the folk-rock anthem's success. The Don & Dewey-cover peaked at #75 on the US Hot 100 on 20 November 1965. It sold around 30,000 copies.

However, it had already been brought to a much larger audience as a track on Sonny & Cher's August 1965 début album 'Look At Us', which spent a staggering eight weeks at #2 on the Billboard 200 and sold over 3-million copies worldwide!